Since Andre Geim and Konstanin Novoselof from University of Manchester in UK successfully stripped pyrolytic graphite out and observed graphene in 2004 (Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666-9), the new carbon materials has been remaining a hot topic in relevant areas. The success of stripping graphene out breaks the prediction about thermal instability of two-dimensional crystal theoretically, and brings about possibilities for more investigations and explorations in new fields.
Perfect graphene is supposed to own ideal two-dimensional structure, which consists of hexagonal lattice. Every single carbon atom is combined with other three carbon atoms by σ bond in the direction of lattice plane, and non-bonding electrons serves as π electrons, forming π orbit system vertical to the lattice plane. The π electron could move randomly in the plane, which enables graphene to own fine electrical conductivity and sustain electric current whose density is six orders of magnitude more than copper. Graphene also owns record-breaking thermal conductivity. The thermal conductivity of pure graphene could reach 2000-4000W·m^(−1)·K^(−1). Graphene also has excellent strength and large surface area. Besides, the special structure of graphene provides unique energy band structure and enables it with half integer quantum hall effect and perfect tunneling effect, as well as electrical conductivity that would never fade away. The special characteristics mentioned above guarantee graphene a promising prospect of application in fields of materials and electronic circuits. Therefore, synthesizing graphene in a large scale is in high demand.
There're two traditional ways to synthesis graphene, which are physical method and chemical method respectively. Properties of graphene obtained through the two methods are different from each other. Physical methods include mechanical stripping, electric arc discharge, ultrasonic dispersion etc. Graphene layers obtained through physical methods are comparatively intact, but there're problems like low productivity, uncertainty of quality, command for special equipment and high cost. Chemical methods include bottom up organic synthesis, oxidation-reduction process, solvothermal synthesis and chemical vapor deposition. Equipment and raw materials are strictly required for organic synthesis method, so it's difficult to achieve mass production in this way. Production quality isn't stable for solvothermal method, thus the average quality is poor. Chemical vapor deposition method costs too high and cannot achieve scale production. Among all those methods, only oxidation-reduction process can work without special equipment, and quality of graphene obtained through this method is stable. Thus it's the most suitable way for industrialized production. During preparation of graphene by redox method, the intermediate of graphene oxide has been involved. Due to various functional groups of graphene oxide, including epoxy hydroxyl, carboxyl, carbonyl etc., it can be dispersed well in an aqueous solution and is capable of forming a stable colloid, so it's easier to chemically modify graphene oxide than graphene. Therefore, the graphene oxide is also a basic raw material for further chemical modification of graphene. However, in any of the preparation process of an oxidized graphene, large amount of strong acid and strong oxidant intercalating agents are consumed and cause corrosion to equipments. Moreover, the process of mixing intercalation agents and strong oxidants often generates a large amount of heat release, which might cause severe reactions between strong oxidizing agent and the intercalation agent, and even explosion. Moreover, sometimes solid material is used as strong oxidant. The inefficient solid-solid reaction occurs between the solid strong oxidant and the graphite dispersed in intercalation agent, and it's difficult to achieve mass production. Therefore, if industrialists use redox method for mass production, several problems (e.g. heat release, equipment corrosion, and reaction rate) need to be solved.
In addition, in the reduction process of graphene oxide, appropriate reducing agents are also critical. Traditionally, graphene oxide reduction is carried out at a neutral or alkaline environment, which causes the agglomeration of graphene oxide, and the reduction reaction would not continue until graphene oxide is dispersed. However, it an acidic environment, all these problems can be avoided and the reduction reaction occurs homogeneously. So, for the mass reduction process, it is very necessary to select appropriate reducing agents that are suitable for the acid environment.